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Background: Venous thromboembolism (VTE) is rare among young women and is often presumed to occur in the setting of a genetic predisposition or during the use of estrogen-containing combined hormonal contraceptive or to have an unknown cause. This study aims to describe the distribution of VTE risk factors among women with a confirmed VTE., Methods: We identified all women aged 15-46 years with a VTE diagnosis at Columbia University Medical Center from 2005 to 2012 using medical center databases. We then reviewed all electronic medical records to validate the diagnoses and identify risk factors associated with each confirmed case., Results: We identified 315 cases and confirmed 186 (59%). The proportion of unconfirmed cases increased over time. Forty percent of confirmed cases were associated with hormonal contraceptives or pregnancy. Ninety-five percent of confirmed cases had identifiable major risk factors including a personal history, family history, malignancy or other predisposing illness, recent long-haul travel, trauma, hospitalization, and obesity; many had multiple simultaneous risk factors. None of the confirmed cases was associated with a previously known genetic predisposition, but in 10 confirmed cases a genetic predisposition was identified during evaluation. In only 10 of the 186 confirmed cases could we not identify any acquired risk factor, and only 2 of those 10 women had a genetic predisposition., Conclusions: Many reproductive age women experiencing a VTE have risk factors unique to this group, and most have multiple risk factors, confirming that this is a multifactorial disease. The large proportion of unconfirmed cases suggests the need for great caution in using administrative databases for research due to poor diagnostic specificity and due to lack of information about additional risk factors.